This invention relates generally to temperature compensation in an electronic circuit and is particularly directed to a temperature compensated oscillator circuit such as employed in a horizontal drive system of a raster scanned video display.
Video information received by a television receiver is presented on a raster which is scanned horizontally at a first rate, and vertically at a second, generally slower rate. The video information is provided in the form of amplitude-modulated synchronizing pulses which are used for synchronizing the raster scan of the television receiver's cathode ray tube (CRT) with the received video information. For proper picture framing, the frequency and phase of oscillation produced by the television receiver's horizontal sweep system must be accurately synchronized with the frequency and phase of a horizontal synchronization signal transmitted from the broadcast station.
In television receivers where a standard input synchronization signal frequency is used, a phase locked loop is utilized to detect the synchronization pulses and to control a horizontal-rate oscillator at a frequency which is an average of the incoming synchronizing signals. In video displays such as used in computer terminals or data display systems where a great variety of input synchronization signals may be encountered, the phase locked loop approach may be replaced by the combination of a monostable multivibrator and a free-running astable multivibrator in the horizontal drive circuit of the video display.
The monostable multivibrator is responsive to a horizontal drive or synchronization input signal for triggering the astable multivibrator in well-defined time relation with respect to the horizontal drive signal. Upon receipt of a synchronization signal, the monostable multivibrator is driven to an unstable state for a predetermined time. A reduction in synchronization input signal amplitude due to an input signal voltage cutoff threshold which reduces spurious multivibrator inputs is compensated for by coupling the synchronization input from the monostable multivibrator to the emitter and collector of both of the astable multivibrator's coupled transistors. This multivibrator combination, by increasing synchronization signal injection to the astable multivibrator, provides an increase in the synchronization signal frequency capture range. An example of this synchronization signal processing arrangement for use in a raster scanned video display can be found in U.S. Pat. No. 4,253,117 which is assigned to the assignee of the present application.
Although representing an improved approach to the synchronization of horizontal sweep with video synchronization input pulses in a raster scanned video display, the free-running astable multivibrator tends to drift in frequency with temperature resulting in a horizontal drift in the video image presented on the faceplate of the CRT. Frequently, a manual adjustment in the form of a potentiometer is provided for compensating for this temperature-dependent drift and permitting the horizontal centering of the video image. However, this requires intervention by the user and is thus somewhat unreliable and inconvenient. In addition, frequently the manual horizontal adjustment means is located within the unit's chassis and is limited for use in the initial set-up and adjustment of the video display unit during its manufacture. This adjustment control is therefore frequently not accessible to, nor intended for use by, a user/viewer of the video display unit.
The prior art discloses various approaches for automatic temperature compensation in an oscillator circuit. One common approach makes use of temperature sensitive components such as a thermistor to compensate for the temperature dependent operating characteristics of other oscillator circuit components. Examples of this approach can be found in U.S. Pat. Nos. 4,380,745 to Barlow et al and 4,412,188 to Helle et al. While perhaps providing a practioal solution to temperature compensation in specialized circuits, the substantial cost of thermistors renders their use somewhat impractical in mass produced, consumer-type products.
The present invention represents an improvement over the prior art by providing a low cost, reliable and accurate temperature compensated oscillator particularly adapted for use in the horizontal drive circuit of a raster scanned video display.